The present invention relates to electromagnetically induced transparency in photonic band-gap structures. More particularly, the present invention relates to the use of physical and modal characteristics of photonic band-gap fibers to support the use of electromagnetically induced transparency in an injected medium.
A photonic crystal is a structure having a multi-dimensional periodic variation of the dielectric constant, resulting in a band gap structure. Electromagnetic wavelengths within the band gap may have reduced transmission, and electromagnetic wavelengths above and below the band gap may have increased transmission.
Introducing defects into the periodic variation of the photonic crystal, resulting in variations of the dielectric constant, can alter allowed or non-allowed light wavelengths which can propagate in the crystal. Light which cannot propagate in the photonic crystal but can propagate in the defect region can be trapped in the defect region. A particular line defect in a three dimensional photonic crystal can act as a waveguide channel, for light wavelengths in the band gap.
Optical waveguide fibers can be generally classified into single-mode fiber and multimode fiber. Both types of optical fiber rely on total internal reflection (TIR) for guiding the photons along the fiber core. Typically, the core diameter of single-mode fiber is relatively small, thus allowing only a single mode of light wavelengths to propagate along the waveguide. Single-mode fibers can generally provide higher bandwidth because the light pulses can be spaced closer together, and are less affected by dispersion along the fiber. Additionally, the rate of power attenuation for the propagating light is lower in a single-mode fiber. Optical fibers which maintain their single mode characteristics for all wavelengths are defined as endlessly single mode fibers.
Optical fibers having a larger core diameter are generally classified as multimode fibers, and allow multiple modes of light wavelengths to propagate along the waveguide. The multiple modes travel at different velocities. This difference in group velocities of the modes results in different travel times, causing a broadening of the light pulses propagating along the waveguide. This effect is referred to as modal dispersion, and limits the speed at which the pulses can be transmitted; in turn limiting the bandwidth of multimode fiber. Graded-index multimode fiber (as opposed to step-index multimode fiber) has been developed to limit the effects of modal dispersion. However, current multimode and graded-index multimode fiber designs do not have the bandwidth capabilities of single-mode fiber.
Photonic crystals are another means by which photons (light modes) can be guided through an optical waveguide structure in multi-mode states. Rather than guiding photons using TIR, photonic crystals rely on Bragg scattering for guiding the light. The characteristic defining a photonic crystal structure is the periodicity of dielectric material along one or more axes. When the dielectric constants of the materials forming the lattice are different; and the material absorbs minimal light, the effects of scattering and Bragg diffraction at the lattice interfaces allow the photons to be guided along or through the photonic crystal structure. FIG. 1 shows a conventional photonic crystal having three photonic band gaps; the allowed frequencies of transmission are above and below each gap.
Electromagnetically induced transparency (EIT) is the effect of making a medium previously opaque or dispersive to an incident laser, partially transparent. A classical explanation is that the electrons in the medium are induced to have little motion at the frequencies of the intended transparent laser light. This can be accomplished by sinusoidal forces offset in phase reducing the motion of the electrons. The motion of the electrons is reflected in the dielectric constant of the medium and reducing the motion effects the dielectric constant at the frequency of interest.
FIG. 2 illustrates one possible example of EIT with respect to three energy state levels, |n greater than , |n+1 greater than , and |n+2 greater than , of an atom in the medium of interest. The atom oscillates between the energy states at the Rabi frequency (the electrons move between energy state levels). Experimentally the coupling laser frequency (wavelength xcexC) is held fixed, while the probing laser frequency (wavelength xcexP) is varied. The transmission of the probing frequency through the medium, having the energy levels shown in FIG. 2, is measured and at particular probing frequencies the transmission increases substantially. The increase in transmission is usually defined by a transparency width within which the probing wavelength enjoys increased transmission. Typically the condition exists that the probing laser""s linewidth (wavelength extent) be small compared to the transparency width.
An example of an electromagnetic induced transparency can be seen in FIGS. 3A and 3B. FIG. 3A illustrates the transmission percentage (%) of a probing laser as a function of the probing laser wavelength, without a coupling laser. FIG. 3B illustrates the transmission % of the probing laser with a coupling laser set to a fixed frequency. With the coupling laser a significant increase in transmission occurs in regions previously devoid of transmission.
Presently the effects of EIT can only be demonstrated under difficult experimental conditions. To obtain a lower resonance line width (the width of the probing laser wavelengths within which transmission increases) the medium must be cooled to near-zero temperatures. Alternatively if the medium is not cooled, a larger resonance line width must be tolerated, which usually requires a large intensity coupling laser.
Exemplary embodiments of the present invention provide methods and devices for the use of electromagnetically induced transparency.
Exemplary embodiments of the present invention provide the use of electromagnetically induced transparency in photonic band gap structures.
Exemplary embodiments of the present invention provide the use of electromagnetically induced transparency in a medium injected into a photonic band gap fiber.
Further areas of applicability of exemplary embodiments of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limited the scope of the invention.